Pharmaceutical compositions comprising rifaximin and amino acids, preparation methods and use thereof

ABSTRACT

The present invention relates to rifaximin compositions comprising amino acids, characterized in that they increase rifaximin solubility in aqueous solutions and are useful in the treatment of disease wherein amino acids and rifaximin are efficacious. The present invention also relates to pharmaceutical compositions comprising rifaximin or one of the pharmaceutically acceptable salts thereof and one or more amino acid(s), wherein the molar ratio between the amino acid(s) and rifaximin is from 1:1 to 10:1, together with pharmaceutically acceptable excipients. The present invention further relates to rifaximin crystals obtained by dissolving rifaximin and amino acids in solutions formed by ethanol/water and evaporating the solution.

FIELD OF THE INVENTION

This invention relates to compositions comprising rifaximin and one ormore amino acids, their method of preparation and their medical use.

BACKGROUND OF THE INVENTION

Rifaximin (INN; see The Merck Index, XIII ed., 8304) is a semi-syntheticnon-aminoglycoside derived from rifamycin. More precisely, it is apyrido-imidazo rifaximin, described and claimed in the Italian patent IT1154655, whereas the European patent EP 0161534 describes a process forits production starting from rifamycin O.

Rifaximin is (S-S, 16Z, 18E, 20S, 21S, 22R, 23R, 24R, 25S, 26S, 27S,28E)-5, 6, 21, 23, 25-pentahydroxy-27-methoxy-2, 4, 11, 16, 20, 22, 24,26-octamethyl-2,7-(epoxy pentadeca-[1,11,13] trienimino)-benzofuro[4,5-e]-pyrido[1,2-(alpha)]-benzimidazole-1,15(2H) dione, 25-acetate),and is represented in FIG. 1.

Rifaximin is also currently sold under the trademark Normix®, Rifacol®and Xifaxan®.

Rifaximin is an antibiotic usually used for local action with a broadspectrum of action against Gram-positive and Gram-negative bacteria andaerobic and anaerobic organisms. Rifaximin has an excellent safetyprofile and it is characterized for a non-systemic absorption.

Rifaximin is used for the treatment of bowel infections caused bynon-enteroinvasive bacteria, traveler's diarrhea, enteritis, dysentery,bowel inflammations such as, for instance, Crohn's disease (CD),ulcerous recto-colitis, irritable bowel syndromes (IBS), paucities,small intestinal bacterial overgrowth (SIBO), diverticular syndromes;pathologies directly or indirectly deriving from bowel infections, suchas for instance hepatic encephalopathy, or which can be used in the pre-and post-operative prophylaxis of bowel infections.

U.S. Pat. No. 4,557,866 describes a process for the synthesis ofpyrido-imidazo rifaximins comprising the reaction of rifamycin O with4-methyl-2-aminopyridine.

EP 1557421 B1, EP 1676847 B1, EP 1676848 B1 and U.S. Pat. No. 7,045,620B2 describe polymorphic forms of rifaximin (INN), called rifaximin α,rifaximin β, and a poorly crystalline form called rifaximin γ. Theseforms can be obtained by hot-dissolving raw rifaximin in ethyl alcoholand by inducing subsequent crystallization of the product by addition ofwater at a given temperature and for a fixed time. The crystallizationis then followed by a drying step carried out under controlledconditions, e.g., until a predefined water content is obtained, and theX-ray diffraction profile corresponds to one observed for one or more ofthe aforesaid rifaximin forms.

These patents also describe processes for the transformation from onepolymorphic form to another, such as obtaining polymorph α bydehydration of polymorph β or polymorph γ; obtaining polymorph γstarting from polymorph α and the preparation of polymorph β byhydration of polymorph α.

U.S. Pat. No. 7,906,542 B2 describes pharmaceutical compositionscomprising polymorphic forms of rifaximin α, β and γ.

EP 1682556 A2 describes polymorphic forms of rifaximin α, β and γ andtheir different in vivo absorption and dissolution profiles.

U.S. Pat. No. 7,915,275 B2 describes the use of pharmaceuticalcompositions comprising polymorphic forms of rifaximin α, β and γ forthe treatment of bowel infections.

WO 2008/155728 describes a process for obtaining amorphous rifaximin byhot-dissolving raw rifaximin in absolute ethyl alcohol and thencollecting after precipitation by cooling rifaximin under amorphousform.

Amorphous forms of rifaximin and processes for their obtainment aredescribed in US 2009/312357 and US 2009/0082558, in particular US2009/0082558 describes that amorphous rifaximin is obtained afterprecipitating by addition of water to a rifaximin solution in organicsolvent.

WO 2009/108730 describes polymorphic forms of rifaximin (form ζ, formγ-1 (ζ), form α-dry, form η, form ι, form β-1, form β-2, form ε-dry),salts, hydrates and amorphous rifaximin, their use in the preparation ofpharmaceutical compositions and therapeutic methods related to theiruse.

WO 2011/153444 describes polymorphic forms of rifaximin κ and θ and WO2011/156897 describes polymorphic forms of rifaximin called APO-1 andAPO-2.

WO 2006/094662 describes polymorphic forms δ and ε of rifaximin usefulin the preparation of pharmaceutical forms for oral and topical use.Said forms are obtained by means of processes comprising hot dissolutionof raw rifaximin in ethyl alcohol, then addition of water atpredetermined temperatures and for predetermined time periods, thendrying under vacuum.

Viscomi et al., Cryst. Eng. Comm., 2008, 10, 1074-1081 describes theprocess for the preparation of polymorphic forms of rifaximin and theirchemical, physical and biological characteristics.

Bacchi A. et al. New Journal of Chemistry (2008), 32; 10; 1725-1735,describe the preparation of crystals of tetra-hydrated rifaximin 1 witha water weight content corresponding to 8.4% (w/w), obtained by slowlyevaporating water/ethanol solution of rifaximin at room temperature.

Rifaximin is a substantially water-insoluble molecule, and organicsolvents are necessary to be added for increasing its solubility inaqueous solutions. Organic solvents are hardly acceptable in thepreparation of substances for pharmaceutical use, and their use requiressevere controls of the residual solvents in the final products.

Rifaximin water solubility can be varied within limited concentrationranges by selecting suitable polymorphic or amorphous forms. Forexample, WO 2005/044823 states that rifaximin polymorph α issubstantially insoluble, whereas WO 2011/107970 states that an amorphousform of rifaximin obtained by means of spray-drying has a solubility ofabout 40 μg/ml after thirty minutes in aqueous solution, but this formis not stable and the solubility decrease over time and after two hoursthe solubility is about 9 μg/ml.

As described by Viscomi et al., Cryst. Eng. Comm., 2008, 10, 1074-1081,rifaximin solubility in suspension the presence of solid rifaximin mayvary during the time according to possible transformation processes inmore stable crystalline forms. In particular, it is described that alsoin case of substantially amorphous rifaximin polymorphs, solubilitydecreases in time until it coincides with the values obtainable with themore stable crystalline forms.

Rifaximin is also a local-action antibiotic, and the in-situbioavailability of pharmaceutical compositions providing for increasedavailable and local rifaximin concentrations (e.g., in physiologicalfluids such as gastric and intestinal fluids) is useful for treating allpathologies for which an increased rifaximin concentration can providehigher therapeutic efficacy.

There is a need in the art for rifaximin formulations having increasedrifaximin solubility in aqueous solutions that provide increasedrifaximin concentrations that are stable with time in comparison tothose obtainable by the prior art.

There is also a need to provide rifaximin pharmaceutical compositionsthat include amino acids for the treatment of all the diseases whereinthe amino acids are efficacious. There is also a need to provide theantibiotic effect of rifaximin with the effect of the amino for thetreatment of hepatic disease and debilitated disease.

There is also a need to obtain compositions providing increasedrifaximin concentrations at room temperature, that may be used directlyin pharmaceutical preparations, in form such as tablets or clearsolutions (replacing granulates in cloudy suspensions that are not welltolerated by patients) or in compositions for vaginal or rectal use.Preferably compositions having rifaximin concentrations higher than 3μg/ml at room temperature would be obtained therefrom.

The provision of rifaximin solutions with increased rifaximinconcentration are convenient for reducing the volumes of solution neededfor use in industrial processes for preparing rifaximin compositionswithout the addition of large volumes of organic solvents.

In particular, rifaximin concentrations having increased solubilitywould be useful in formulating gastroresistant compositions promotingthe release of high concentrations of rifaximin in the intestine for thetreatment of bowel infections.

In the prior art, rifaximin can be obtained in powder, in raw form, inpolymorphic or amorphous forms.

The information concerning the crystalline characteristics of rifaximinpolymorphs available in the prior art has been obtained by means of theX-ray powder diffraction techniques. The obtained powder diffractogramsare the result of the contribution of several micro-crystals (orcrystallites) forming the powder; the observed powder diffractionsignals which are often broadened and have a non-constant intensity,even re-analyzing the same sample, since the signals can be influencedby several factors, such as, for instance, the size and morphology ofthe crystallites and their distribution in the sample holder. Therefore,the univocal attribution to a settled phase of a water content as wellas of an exact proportion of possibly present solvates and/or hydratesby means of X-ray powder diffraction can be rather difficult.

Generally, the crystal size and structure influences some properties ofthe powder of an active principle. For example, Kiang Y H et al., Int.J. Pharm., 368 (2009, 76) reports that mechanical properties, such ascompressibility and flowability, are related to crystal morphology(structure) and that these properties influence the preparation offinished compositions in solid form.

Vippagunta S. R. et al., Adv. Drug. Del. Rev. 48 (2001), 3-26, discussesthe relevance of controlling the crystalline (i.e., polymorphic) formsof an active principle during the various stages of its development,because each phase change due to interconversion of the polymorphs, tosolvation processes, to hydrate formation and to change of crystallinitydegree can alter the bioavailability of the drug.

The correlation between solid structure (i.e., morphology) andpharmacologically useful properties, such as bioavailability, isrecognized as relevant information to be considered during the drugapproval process. In fact, for giving their approval to thecommercialization of drugs, health authorities require suitableanalytical techniques for identifying the crystalline structure of theactive principle, as well as production processes of the finishedproduct for obtaining consistent amounts of the specific polymorphicforms. For example, the European Medicines Agency that regulates thegranting of marketing authorization of drugs requires that themanufacturing methods of the active ingredients are standardized andcontrolled in such a way that they give homogeneous and sound results interms of polymorphism of production batches (see, CPMP/QWP/96, 2003—Notefor Guidance on Chemistry of new Active Substance; CPMP/ICH/367/96—Notefor guidance specifications: test procedures and acceptance criteria fornew drug substances and new drug products: chemical substances; Date forcoming into operation: May 2000).

Therefore, the availability of sufficiently pure, high quality crystalsof a particular polymorph or solvate (including hydrates) of a suitablesize is critical and quite useful for providing analytical standardsthat enable the identification of single polymorphs present in mixtures.

The availability of the above described analytical standards is highlyrelevant to the pharmaceutical arts, for example, such analyticalstandards are useful for the identification of the polymorphic formspresent, e.g., in a mixture, and for the identification of a particularspecies of solvate (e.g., hydrates) characterized by the stoichiometricratio of water to active principle. It is well known that the presenceor the absence of solvent molecules (e.g., water) in specificcrystallographic positions can have an influence on the position of thepeaks in a powder diffractogram and, in the case of rifaximin, knowingsuch positions would allow a better interpretation of thesediffractograms.

Also, the availability of crystals suitable for analysis via singlecrystal X-ray diffraction enables the identification of individualpolymorphs (and solvates thereof) in complex mixtures and also the exactwater content of a polymorph could be determined thanks to theinformation provided by such technique. In particular, the quantitativecharacterization of a mixture including an amorphous form of thecompound is difficult because the amorphous form does not give specificsignals in a diffractogram, but instead is detectable by the presence ofa raised baseline in the powder diffractogram. The availability ofsingle crystal X-ray diffraction data as an analytical standardcorresponding to the polymorphs and/or solvates present in the mixture,allows the quantification of the amount of amorphous substance in amixture.

A better understanding of the crystalline structure is also relevant forthe preparation of reproducible pharmaceutical compositions. Forexample, production processes can be modified in order to obtaincompounds with reproducible crystallinity, thus guaranteeing thepresence of properties corresponding to particular crystallinity relatedto particular polymorphic forms.

SUMMARY OF THE INVENTION

An embodiment of the present invention is provided by compositionscomprising a mixture rifaximin and amino acids, wherein amino acids andrifaximin are in a molar ratio from 1:1 to 10:1, said compositionsenabling the preparation of rifaximin crystals and useful for thepreparation of pharmaceutical compositions.

An embodiment of the present invention are rifaximin crystals obtainedfrom compositions of rifaximin and amino acids dissolved in aqueoussolutions, and the solution is evaporated. The obtained crystals areuseful as analytical standards. A further embodiment of the presentinvention is the use of amino acids for increasing rifaximinconcentrations in solution, in comparison to those of the state of theart.

In particular, the effect of amino acids is such that rifaximinconcentrations in water of higher than 3 μg/ml at room temperature andhigher than 7 μg/ml at 37° C. can be obtained.

Embodiments of the present invention also show that amino acids have asynergic effect together with organic solvents in increasing rifaximinsolubility in aqueous solutions containing low percentages of organicsolvents.

The use of amino acids in combination with rifaximin, according toembodiments of the present invention, provides novel compositions withrifaximin, enables the production of rifaximin crystals, e.g., suitablefor analysis by single crystal x-ray diffraction, increases rifaximinconcentration in aqueous solutions and provides for the preparation ofpharmaceutical compositions.

Embodiments relate to pharmaceutical compositions comprising rifaximinor one of the pharmaceutically acceptable salts thereof and one or moreamino acid(s), wherein the molar ratio between the amino acid(s) andrifaximin is from 1:1 to 10:1, preferably from 1:1 to 5:1, together withpharmaceutically acceptable excipients, which provide increasedrifaximin solubility in aqueous solution.

In some embodiments, the pharmaceutical compositions comprisepharmaceutically acceptable ingredients that include diluting agents,binding agents, disintegrating agents, lubricating agents,release-controlling polymers or bioadhesive polymers.

Embodiments further relate to a process for preparing saidpharmaceutical compositions comprising the steps

-   -   mixing rifaximin and amino acids;    -   adding the excipients and mixing the final mixture in, for        example, a V mixer for a time from 10 to 30 minutes and    -   granulating the product in, for example, a roller compactor.

Embodiments further relate to the use of one or more amino acids forobtaining a pharmaceutical composition having a rifaximin concentrationfrom 4.5 μg/ml to 60 μg/mi.

Embodiments further relate to rifaximin crystals characterized in thatthey are obtained by means of a process comprising:

-   -   a) dissolving rifaximin and amino acids, wherein the amino acids        and rifaximin are in a molar ratio from 1:1 to 10:1 in a        solution of ethanol/water, in a volumetric ratio from 1:1 to        1:10 (v/v);    -   b) evaporating the solution obtained in step a) at temperatures        from room temperature to 40° C., in a time period from 1 to 10        days;    -   wherein the resulting crystals have monodinic space group P2₁        and cell parameters in the ranges: a: 13.7(1)-13.8(1) Å; b:        19.7(1)-19.9(1) Å; c: 16.4(6)-16.6(6) Å; β: 92.1(1)-91.9(1) deg.

Embodiments further relate to a process for the production of rifaximincrystals, wherein the process comprises:

-   -   a) dissolving rifaximin and amino acids, wherein the amino acids        and rifaximin are in a molar ratio from 1:1 to 10:1 1, in        solutions of ethanol/water, in a volumetric ratio from 1:1 to        1:10 (v/v);    -   b) evaporating the solution obtained in step a) at temperatures        from room temperature to 40° C., in a time period from 1 to 10        days.

In some embodiments, the resulting crystals are characterized bymonoclinic space group P2₁ and cell parameters in the ranges: a:13.7(1)-13.8(1) Å; b: 19.7(1)-19.9 (1) Å; c: 16.4(6)-16.6(6) Å; β:92.1(1)-91.9(1) deg.

Embodiments further relate to a process for the production of rifaximincrystals, wherein the process comprises:

-   -   a) dissolving rifaximin and amino acids, wherein the amino acids        and rifaximin are in a molar ratio from 1:1 to 10:1, in solution        of ethanol/water, in a volumetric ratio from 1:1 to 1:10 (v/v);    -   b) evaporating the solution obtained in step a) at temperatures        from room temperature to 40° C., in a time period from 1 to 10        days, in the presence of dehydrating agents.

In some embodiments, the resulting crystals have monoclinic space groupP21 and cell parameters in the ranges: a: 14.2(1)-14.5(1) Å; b:19.7(1)-20.1(1) Å; c: 16.1(1)-16.2(1) Å; β: 108.7(1)-111.4(1) deg.

Embodiments further relate to the use of amino acids and rifaximin in amolar ratio from 1:1 to 10:1 in order to obtain rifaximin crystals. Insome embodiments, the rifaximin crystals are characterized by monodinicspace group P21 and cell parameters in the ranges: a: 13.7(1)-13.8(1) Å;b: 19.7(1)-19.9(1) Å; c: 16.4(6)-16.6(6) Å; β: 92.1(1)-91.9(1) deg. Insome embodiments, the rifaximin crystals are characterized by monoclinicspace group P21 and cell parameters in the ranges: a: 14.2(1)-14.5(1) Å;b: 19.7(1)-20.1(1) Å; c: 16.1(1)-16.2(1) Å; β: 108.7(1)-111.4(1) deg.

The crystalline form of rifaximin in the pharmaceutical compositionsaccording to the foregoing embodiments may be at least one selectedfrom:

-   -   i) crystals having monoclinic space group P2₁ and cell        parameters in the ranges: a: 13.7(1)-13.8(1) Å; b:        19.7(1)-19.9(1) Å; c: 16.4(6)-16.6(6) Å; β: 92.1(1)-91.9(1)        deg.,    -   ii) crystals having the features of i) and having 3 or 4.5 water        molecules for each rifaximin molecule,    -   iii) crystals having monoclinic space group P21 and cell        parameters in the ranges: a: 14.2(1)-14.5(1) Å; b:        19.7(1)-20.1(1) Å; c: 16.1(1)-16.2(1) Å; β: 108.7(1)-111.4(1)        deg.    -   iv) crystals having the features of iii) and having zero or 0.5        or 1.5 water molecules for each rifaximin molecule, or    -   v) rifaximin α, β, γ, δ.

In some embodiments, the compositions comprise rifaximin in the form ofhydrate, solvate, polymorphous, amorphous or crystalline form ormixtures thereof.

These compositions are useful for treating and preventing inflammatoryand infection diseases susceptible to rifaximin treatment.

The compositions of the invention comprise one or more amino acidswherein amino acids are aliphatic amino acids, aromatic amino acids,basic amino acids, branched amino acids, cyclic amino acids, acidicamino acids, hydroxyl or sulfur containing amino acids, amide aminoacids, or mixtures thereof.

In some embodiments, the compositions of the invention comprise one ormore aromatic or heterocyclic amino acids.

In one particular aspect the composition comprise a mixture of one ormore aromatic or heterocyclic amino acids and branched amino acids.

In some embodiments, the compositions comprise valine, leucine, orisoleucine in a molar ratio relative to rifaximin of 10:1.

Pharmaceutical compositions can comprise rifaximin in a dosage amount offrom 20 mg to 1200 mg with amino acids in a molar ratio relative torifaximin from 1:1 to 10:1, preferably from 1:1 to 5:1. Suchcompositions can be administered in a dosage range of from 20 to 3000 mgper day.

Pharmaceutical compositions can be in the form of a powder, paste,granulates, tablets, capsules, pessaries, cream, ointment, suppository,suspension or solution. They can be suitable for human or animal use.

Pharmaceutical compositions can comprise amino acids and rifaximin inform of a homogenously mixed powder or in the form of “conglomerate”,wherein the term conglomerate indicates rifaximin crystals and aminoacids, which are generated to form a more cohesive mass betweenrifaximin and amino acids.

In some embodiments, conglomerates are obtained when amino acids andrifaximin, in a molar ratio from 1:1 to 10:1, preferably from 1:1 to5:1, are solubilized in aqueous solutions in the presence of organicsolvent, preferably alcohol, in a volumetric ratio from 5% to 25% attemperature between ambient temperature and boiling temperature, whereinthe solvent is subsequently evaporated.

Embodiments are also directed to pharmaceutical compositions comprisingrifaximin conglomerates and acceptable excipients, wherein thecompositions provide increased rifaximin solubility with respect to thatof the prior art.

Embodiments also relate to conglomerates of rifaximin which compriserifaximin crystals.

The rifaximin crystals, obtained by conglomerate, are in a form ofcrystal and in particular in form of a single crystal suitable for X-raydiffraction and which can characterized by defined crystal parametersand water contents.

Embodiments also relate to processes for the preparation of thepharmaceutical compositions comprising one or more amino acids andrifaximin, wherein the amino acids are in a molar ratio from 1:1 to 10:1for the preparation of solid forms as tablets, granulates, ointments,cream, suppositories and solution. The processes comprise: (a)dissolving rifaximin and one or more amino acids, wherein amino acidsand rifaximin are in a molar ratio from 1:1 to 10:1, in solutions ofethanol/water, in a volumetric ratio from 1:1 to 1:10 (v/v); and (b)evaporating the solution obtained in step a). In some embodiments,evaporation of the solution takes place at temperatures from roomtemperature to 40° C. In some embodiments, evaporation of the solutiontakes place during a time period from 1 to 10 days. In some embodiments,evaporation of the solution takes place in the presence of a dehydratingagent.

Embodiments also relate to the use of pharmaceutical compositionscomprising rifaximin and one or more amino acids for treatment or in theprevention of infections and disease in an animal or human in needthereof, wherein provision of rifaximin and amino acid(s) areefficacious.

In some embodiments, the composition comprising amino acids andrifaximin is provided, for example, by administering the composition,for the treatment or prevention of traveler's diarrhea, hepaticencephalophathy, infectious diarrhea, diverticular disease, as anantibacterial prophylactic prior to and post colon surgery, irritablebowel syndrome, Crohn's disease, Clostridium difficile-associateddiarrhea, small intestinal overgrowth, traveler's diarrhea prophylaxis,dysentery, pauchitis, peptic ulcer disease, surgical prophylaxis andgastric dyspepsia. In some embodiments, the composition is provided toan animal or human in need thereof, for treatment or prevention of bowelinfections, diarrhoea, irritable bowel syndrome, bacterial growth insmall intestine, Crohn's disease, hepatic insufficiency, hepaticencephalopathy, enteritis and fibromyalgia.

In some embodiments, the pharmaceutical compositions comprise at least abranched amino acid beneficial in the treatment of hepatic disease suchas hepatic encephalopathy and complication of cirrhosis disease.

The pharmaceutical compositions disclosed herein can provide increasedlocal concentrations of rifaximin while providing an energetic andnourishing effect of amino acids that provides a beneficial effect forthe patient.

Embodiment also relate to the use of amino acids for increasingrifaximin solubility, also in the presence of small volume of organicsolvents. For example, the organic solvent can be present in an amountof from 1 to 20% (v/v).

Embodiments also relate to methods of determining the amount ofrifaximin crystals in a sample of rifaximin, comprising obtaining asample of rifaximin; carrying out a crystallographic analysis of thesample; and comparing the resulting analysis of the sample with acrystallographic analysis obtained with rifaximin crystals obtained asdescribed herein. Such analysis may be used in the manufacture ofpharmaceutical compositions for quality control, and monitoring andadjusting the amount of rifaximin in pharmaceutical composition toprovide the appropriate therapeutic dose.

Embodiments also relate to methods of determining the presence of arifaximin polymorph in a sample of rifaximin, comprising obtaining asample of rifaximin; carrying out a crystallographic analysis of thesample; and comparing the resulting analysis of the sample with acrystallographic analysis obtained with rifaximin crystals obtained asdescribed herein.

Other embodiments are described infra.

DETAILED DESCRIPTION OF THE INVENTION

The object of the present invention is represented by rifaximincompositions comprising one or more amino acids and rifaximin, whereinthe one or more amino acids and rifaximin are in a molar ratio from 1:1to 10:1, preferably from 1:1 to 5:1, respectively.

The compositions can be in solid form or in aqueous solutions.

Pharmaceutical compositions comprising rifaximin and one or more aminoacids, wherein the amino acids are in a molar ratio to rifaximin fromabout 1:1 to 10:1, preferably from about 1:1 to 5:1, can provideincreased rifaximin solubility on an order of magnitude from about 1.5-to 20-fold in aqueous solutions at room temperature relative to that ofsolutions that do not contain an amino acid. In some embodiments, suchcompositions can provide increased rifaximin solubility on an order ofmagnitude from about 1.1- to 10-fold at 37° C. relative to that ofsolutions that do not contain an amino acid. The increased rifaximinsolubility can vary according to the chosen amino acid.

The increase in rifaximin solubility in the presence of amino acids isobserved when the compositions comprise rifaximin in the form of rawrifaximin, amorphous rifaximin, a rifaximin polymorph, or mixturesthereof.

When amino acids are in solution with low volumetric percentages oforganic solvents, in particular from 1% to 25% (V/V), a synergic effectresulting from the presence of amino acids and organic solvents on theincreasing of rifaximin solubility is observed and the solubilityincreases up to thousand time reaching concentrations higher than 30mg/ml.

The synergistic effect of the one or more amino acids in combinationwith organic solvents on aqueous rifaximin solubility is alsoadvantageous for the manufacturing process of the pharmaceuticalcompositions described herein. For example, the use of organic solventduring the manufacturing process can be avoided or reduced. In addition,the synergistic effect provided by amino acids in the presence of smallvolumes of organic solvent can provide solutions with higherconcentrations of rifaximin, which can be used to form conglomerates ofrifaximin and amino acids as described herein.

The one or more amino acids present in rifaximin compositions of thepresent invention can be an aliphatic amino acid, such as, for example,glycine or alanine; a branched amino acid, such as, for example, valine,leucine or isoleucine; a hydroxyl or sulfur containing amino acid, suchas, for example, cysteine, threonine or methionine; a cyclic amino acid,such as, for example, proline; a heterocyclic amino acid, such as, forexample, proline or histidine; an aromatic amino acid, such as, forexample, phenylalanine, tyrosine or tryptophan; a basic amino acid, suchas, for example, histidine, lysine or arginine; an acidic amino acid,such as, for example, aspartic acid or glutamic acid; an amide aminoacid such as, for example, asparagine or glutamine; or mixtures thereof.

The use of different amino acids leads to different rifaximinconcentrations, thus, it is possible to modulate rifaximin solubility inan aqueous solution, for use directly or for use in manufacturing apharmaceutical composition, including solid compositions, for differentdiseases.

It has been found that aromatic amino acids or amino acids comprising aheterocyclic ring provide higher solubility of rifaximin in aqueoussolutions.

In particular it has been found that the presence of tryptophan and/orhistidine provides increased solubility of rifaximin.

Compositions of the present invention comprise rifaximin in the form ofa hydrate, a solvate, a polymorphous form, amorphous form, or mixturesthereof, and one or more amino acids, wherein the one or more aminoacids and rifaximin are present in a molar ratio preferably from about1:1 and 10:1, more preferably from about 1:1 and 5:1. Such compositionscan be formulated with one or more pharmaceutically acceptableexcipients for the preparation of pharmaceutical compositions in solidor liquid form.

In humans and animals, amino acids have important roles as metabolicintermediates. When administered to humans, the amino acids either areused to synthesize proteins or other biomolecules, or they are oxidizedto urea and carbon dioxide as a source of energy.

Amino acids also have an energetic and nourishing effect, therefore, theuse of amino acids in pharmaceutical compositions comprising rifaximinfor the treatment of all pathologies that are susceptible to treatmentwith rifaximin with related debilitative diseases can have a beneficialeffect on a patient in need thereof.

Compositions comprising one or more amino acids are described herein. Ina preferred embodiment, compositions comprising aromatic amino acid andbranched amino acids such as leucine, isoleucine and valine are usefulfor increasing solubility of rifaximin as well as for providing abeneficial effect in the treatment of hepatic encephalopathy. Thesecompositions have the advantage of providing a higher availableconcentration of rifaximin relative to that of the prior art in thetreatment of hepatic encephalopathy.

The compositions comprising rifaximin and amino acids of the inventionalso have the advantage of providing for an increased amount of solublerifaximin and therefore a higher concentration of rifaximin available atthe site of action, thus allowing better efficacy and/or total amount ofdrug administered, and depending on the form, locally or moresystemically. In particular, the addition of amino acids to rifaximin ingastroresistant and/or controlled release compositions allows therelease of higher rifaximin concentrations in the intestinal tract wherean infection is localized.

Compositions of the present invention can also comprise rifaximin andone or more amino acids in the form of conglomerates of rifaximin andamino acids, wherein the term “conglomerate” refers to a solid materialobtained by drying an aqueous solution of rifaximin and amino acids. Theconglomerates of the present invention are characterized by the presenceof crystals of rifaximin and crystals of one or more amino acids whereinthe rifaximin crystals have suitable size and quality to be analyzed bysingle crystal x-ray diffraction. Preferably, one or more crystals ofrifaximin in the conglomerates are characterized by the dimensions from0.1 mm to 0.3 mm×from 0.1 mm to 0.3 mm×from 0.1 mm to 0.3 mm. Theconglomerates of the invention comprising amino acids and rifaximin canbe obtained by drying aqueous solutions having a molar ratio of aminoacids to rifaximin ranging from 1:1 to 10:1. In some embodiments, theaqueous solutions are dried by evaporation. Pharmaceutical excipientscan be added to these conglomerates for the preparation of a desiredform and further, to provide single rifaximin crystals, characterized byforms of pure crystal.

The rifaximin crystals, so obtained, are useful as analytical standards.

The amino acids-rifaximin compositions can also comprise conglomeratesof rifaximin with acceptable pharmaceutical excipients. The preparationof such pharmaceutical compositions of the invention comprise the stepsof obtaining a conglomerate of rifaximin and one or more amino acids andthe step of combining the conglomerate with pharmaceutically acceptableexcipients.

Compositions comprising rifaximin and one or more amino acids, orconglomerates containing the same, can be in the form of granules whichmay optionally be coated with one or more controlled release agents. Thegranules, together with extragranular excipients, can be used for thepreparation of pharmaceutical compositions. The term “acceptableingredients” includes pharmaceutically acceptable materials,compositions or vehicles, such as liquid or solid fillers, diluents,excipients, solvents or encapsulating material suitable for human oranimal use.

Diluting agents, disintegrating agents, lubricating agents, polymers forconferring gastroresistance or controlled release are included, forexample, among the excipients useful in the preparation ofpharmaceutical compositions.

Exemplary diluting agents useful in the preparation of pharmaceuticalcompositions described herein can include, but not be limited to, atleast one selected from the group consisting cellulose, microcrystallinecellulose, calcium phosphate, starch, kaolin, dehydrated calciumsulphate, calcium carbonate, lactose, saccharose, glucose, sorbitol andmannitol

Exemplary binding agents useful in the preparation of pharmaceuticalcompositions described herein can include, but not be limited to, atleast one selected from the group consisting of cellulose, cellulosederivatives, carboxy methyl cellulose, microcrystalline cellulose,hydroxy propyl cellulose, hydroxy ethyl cellulose, hydroxy propyl-methylcellulose, starches, potato starch, maize starch, partially gelatinizedstarch, gums, synthetic gum, natural gums, polyvinyl pyrrolidone,polyethylene glycol, gelatin, polyols, propylene glycol, alginates andsugars.

Exemplary disintegrating agents useful in the preparation ofpharmaceutical compositions described herein can include, but not belimited to, at least one selected from the group consisting of sodiumcarboxy methyl cellulose (also called carmelose sodium), cross-linkedsodium carboxy methyl cellulose (also called croscarmelose sodium),polyvinyl pyrrolidone (also called povidone), cross-linked polyvinylpyrrolidone (also called crospovidone), starch, pre-gelatinized starch,and silica. Exemplary lubricating agents useful in the preparation ofpharmaceutical compositions described herein can include, but not belimited to, at least one selected from the group consisting of silica,magnesium stearate, calcium stearate, sodium stearyl fumarate,hydrogenated vegetable oils, mineral oils, polyethylene glycols, sodiumlauryl sulphate, glycerides, sodium benzoate, glyceryl dibehenate andglycerol stearate.

The polymers suitable for obtaining controlled release can have asynthetic or natural origin. Exemplary polymers suitable for thepreparation of the pharmaceutical compositions described herein caninclude, but not be limited to, at least one selected from the groupconsisting of copolymers of acrylic acid, such as the copolymermethacrylic acid-ethyl acrylate 1:1, copolymers of methacrylic acid withan acrylic or methacrylic ester such as the copolymer methacrylicacid-ethyl acrylate 1:1 and the copolymer methacrylic acid-methylmethacrylate 1:2, polyvinyl acetate phthalate, hydroxy propyl methylcellulose phthalate, cellulose acetate phthalate, commercially availableproducts, for instance with the trademarks Kollicoat®, Eudragit@,Aquateric®, Aqoat®; natural polymers like shellac, commerciallyavailable with the trademark Aquagold® (shellac 25%) and ethylcellulose.

The pharmaceutical compositions described herein can also havebioadhesive properties in order to adhere to intestinal mucosa. Examplesof polymers, oligomers or their mixtures which can confer bioadhesiveproperties can include at least, but not be limited to, one selectedfrom the group consisting of: pectins, zeins, casein, gelatin, albumin,collagen, kitosan, oligosaccharides and polysaccharides such as, forinstance, cellulose, dextran, polysaccharides from tamarind seeds,xanthan gum, arabic gum, hyaluronic acid, alginic acid and sodiumalginate.

When the bioadhesive polymer is a synthetic polymer, the polymer can be,but not limited to, at least one selected from the group consisting ofpolyamides, polycarbonates, polyalkylenes, polyalkylene glycols,polyalkylene oxides, polyalkylene terephthalates, polyvinyl alcohols,polyvinyl ethers, polyvinyl esters, polyvinyl pyrrolidone,polysiloxanes, polyurethanes, polystyrenes, polymers of acrylic acid andmethacrylic esters, copolymers of methacrylic acid-ethyl acrylate,polylactides, barbituric polyacids, polyanhydrides and polyorthoesters.

Further useful polymers include, for example, methyl cellulose, ethylcellulose, hydroxy propyl cellulose, hydroxy butyl methyl cellulose,cellulose acetate, cellulose propionate, cellulose acetate butyrate,cellulose acetate phthalate, carboxy methyl cellulose, cellulosetriacetate, cellulose sulfate sodium salt, polymethyl methacrylate, polyisobutyl acrylate, poly octadecyl acrylate, polypropylene, polyethyleneglycol, polyethylene oxide, polyethylene terephthalate, polyvinylacetate, polyvinyl chloride, polystyrene, polyvinyl pyrrolidone,polyvinyl phenol and mixtures thereof.

Another group of polymers useful in the obtainment of bioadhesivity arepolymers having a branch with at least one bonded hydrophobic group,wherein hydrophobic groups generally are non-polar groups. Examples ofsaid hydrophobic groups can include at least, but not be limited to, oneselected from the group consisting of alkyls, alkenyls and alkyl groups.Preferably, hydrophobic groups are chosen to increase polymersbioadhesivity. Other polymers are characterized by hydrophobic brancheswith at least one hydrophilic group, such as carboxylic acids, sulphonicacids and phosphonic acids, neutral and positively charged amines,amides and imines, wherein the hydrophilic groups are such to increasethe polymer bioadhesivity.

Pharmaceutical compositions comprising rifaximin and amino acids canoptionally comprise also edulcorating agents, coloring agents,anti-oxidizing agents, buffering agents and flavoring agents.

Exemplary edulcorating/sweetening agents useful in the preparation ofpharmaceutical compositions described herein can include, but not belimited to, at least one selected from the group consisting of potassiumacesulfame, sorbitol, mannitol, isomalt, maltitol, lactitol, xylitol,aspartame, cyclamic acid, cyclamate salts, lactose, sucralose,saccharine and saccharine salts.

When the amino acids—rifaximin compositions are administered aspharmaceuticals to humans and animals, they can be given without otherexcipients, or as a pharmaceutical composition containing, for example,from 0.1 to 90% of active ingredient in combination with one or morepharmaceutically acceptable excipients, such as a carrier.

The pharmaceutical compositions comprising rifaximin and one or moreamino acids, together with pharmaceutically acceptable excipients, canbe in the form of granulates, tablets, capsules, creams, ointments,suppository, suspensions or solutions of rifaximin suitable for humanand/or animal administration.

The pharmaceutical compositions comprising rifaximin and one or moreamino acids can include rifaximin in an amount from 20 to 1200 mg, forexample, 30, 40, 50, 100, 200, 400, 600, 800 and 1000, preferably from100 to 600 mg. Such compositions are useful in the prevention andtreatment of, for example, traveler's diarrhea, hepatic insufficiency,hepatic encephalopathy, infectious diarrhea, diverticular disease, anantibacterial prophylactic prior and post colon surgery, irritable bowelsyndrome, Crohn's disease, Clostridium difficile-associated diarrhea,small intestinal overgrowth, traveler's diarrhea prophylaxis, dysentery,pauchitis, peptic ulcer disease, surgical prophylaxis, gastric dyspepsiaenteritis, fibromyalgia, vaginal infections and as an antibacterialprophylactic prior to and/or post colon surgery.

Actual dosage levels and time course of administration of the activeingredients in the pharmaceutical compositions may be varied so as toobtain an amount of the active ingredient which is effective to achievethe desired therapeutic response for a particular subject, composition,and mode of administration, without being toxic to the subject. Anexemplary dose range is from 20 to 3000 mg per day. Other doses include,for example, 600 mg/day, 1100 mg/day and 1650 mg/day. Other exemplarydoses include, for example, 1000 mg/day, 1500 mg/day, from 500 mg toabout 1800 mg/day or any value in-between.

The amino acids amount is typically in a molar ratio from 1 to 10 withrespect to the rifaximin, preferably in a molar ratio from 1 to 5. Inembodiments in which the amino acids are branched amino acids, theamount of amino acid present in the composition is preferably in a molarratio of from 1 to 10 with respect to the rifaximin.

The pharmaceutical compositions can be formed by various methods knownin the art, such as, for example, granulation, direct compression ordouble compression. In a preferred embodiment, the processes for makinga pharmaceutical composition, wherein amino acids are in a molar ratiofrom 10:1 to 5:1 with respect to rifaximin, comprise the steps of mixingrifaximin and one or more amino acids to obtain a homogeneous mixtureand adding excipients for preparation of the composition in solid orliquid form for oral administration. Suitable forms include, for exampletablets, powder, granules, pastes and capsules. Suitable modes ofadministration include parenteral administration, for example, bysubcutaneous, intramuscular or intravenous injections of, for example, asterile solution or suspension; topical application, for example, as acream, ointment or spray applied to the skin; intravaginally orintrarectally, for example, as a pessary, cream, a foam; or an aerosol.

Conglomerates of rifaximin and amino acids allow for stable storage ofrifaximin without any rifaximin transformation as well as being usefulfor obtaining single crystals of rifaximin. he preparation of rifaximincrystals in the conglomerates comprises mixing rifaximin and one or moreamino acids in aqueous solution and complete evaporation of solvent atroom temperature or at temperatures between room temperature and 100° C.The drying step can take place in the presence or absence of dehydratingagents and can occur at ambient pressure or under vacuum. The resultingcrystals are analyzed and characterized by having high purity with sizesthat are suitable for structure characterization via single crystalX-ray diffraction. The crystals are parallel-piped or cubed, andpreferably their dimension are from 0.1 mm to 0.3 mm×from 0.1 mm to 0.3mm×from 0.1 mm to 0.3 mm.

The compositions of rifaximin and amino acids allow to obtain rifaximincrystals. These latter are obtained when the compositions of rifaximinand amino acids contain amino acids and rifaximin in a molar ratio from1:1 to 10:1, preferably from 1:1 to 5:1, wherein the amino acids andrifaximin are dissolved in aqueous solutions comprising alcohols in avolumetric water/alcohol ratio from 1:1 to 10:1 (v/v), preferably about5:1 (v/v), wherein the rifaximin concentration in solution is higherthan 15 mg/ml.

In some embodiments, rifaximin crystals are obtained by means of aprocess comprising a evaporation of the aforesaid solutions of rifaximinand amino acids at room temperature or at temperatures from roomtemperature to 100° C., followed by a drying step, which be in thepresence or absence of dehydrating agents. The resulting crystals arecharacterized by having high purity and by being a sufficient size forbeing analyzed by means of single crystal X-ray diffraction.

The rifaximin comprised in the compositions for formation of rifaximincrystals can be raw rifaximin, amorphous rifaximin, pure rifaximinpolymorphs, and solvates or mixtures thereof. The compositions mayinclude the presence of single amino acids or mixtures thereof.

Rifaximin crystals obtained as described herein have been characterizedby means of the X-ray diffraction technique, which provide, for example,information about cell parameters and structural details (atomiccoordinates, connectivity, distances and bond angles). The rifaximincrystals analyzed by X-ray diffraction were characterized as having cellparameters a, b, c, and α, β and γ within the ranges reported in Table1.

TABLE 1 Values of cell parameters a/Å 13.7(1)-13.8(1) b/Å19.7(1)-19.9(1) c/Å 16.4(6)-16.6(6) α/deg 90 β/deg 92.1(1)-91.9(1) γ/deg90

In particular, rifaximin crystals obtained by the methods describedherein are characterized by the following cell parameters:

-   -   crystal 1: a: 13.7960(8) Å; b: 19.944 (4) Å; c: 16.607(6) Å; β:        92.180(1) deg; α and γ: 90 deg;    -   crystal 2: a: 13.753(8) Å; b: 19.749 (4) Å; c: 16.378(6) Å; β:        91.972(1) deg; α and γ: 90 deg.

Knowledge of cell parameters and structural details allows thecalculation of the theoretical diffractogram, which is then compared tothe experimental diffractograms obtained from the powder samples ofrifaximin.

This comparison shows that the crystals described in Table 1, identifiedas crystals “1” and “2” are crystals of rifaximin 3. Crystal 1 containsstoichiometrically 3 water molecules for each rifaximin molecule, and iscalled rifaximin 33.0 (where the subscript indicates the number of watermolecules for each rifaximin molecule); crystal 2 containsstoichiometrically 4.5 water molecules for each rifaximin molecule, andis called rifaximin β_(4.5).

Other rifaximin crystals have been obtained by means of drying ofsolutions comprising rifaximin and one or more amino acids, or by meansof direct drying (e.g. in the presence of P₂O₅) of crystals of rifaximin1, wherein the size of the formed crystals is large enough to beanalyzed by single crystal X-ray diffraction.

Such rifaximin crystals, analyzed by means of single crystal X-raydiffraction, are characterized as having cell parameters a, b c and α, βand γ comprised in the ranges reported in Table 2.

TABLE 2 Values of cell parameters a/Å 14.2(1)-14.5(1) b/Å19.7(1)-20.1(1) c/Å 16.1(1)-16.2(1) α/deg 90 β/deg 108.7(1)-111.4(1)γ/deg 90

In particular, rifaximin crystals have been found which arecharacterized by the following cell parameters:

-   -   crystal 3: a: 14.232(4) Å; b: 19.822 (4) Å; c: 16.164(4) Å; β:        108.74(3) deg; α and γ: 90 deg;    -   crystal 4: a: 14.579(4) Å; b: 20.232 (4) Å; c: 16.329(4) Å; β:        111.21(3) deg; α and γ: 90 deg;    -   crystal 5: a: 14.492(4) Å; b: 20.098 (4) Å; c: 16.215(4) Å; β:        111.21(3); α and γ: 90 deg.

Comparison of the theoretical diffractogram of the crystals with theexperimental diffractogram obtained from the powder samples of rifaximinindicate that the crystals previously described in Table 2 (e.g. crystal3, crystal 4 and crystal 5) are crystals of rifaximin α.

In particular, crystal 3 does not contain any water molecule, and iscalled α0; crystal 4 contains stoichiometrically 0.5 water molecules foreach rifaximin molecule and is called rifaximin α_(0.5); crystal 5contains stoichiometrically 1.5 water molecules for each rifaximinmolecule and is called rifaximin α_(1.5).

Rifaximin single crystals as described herein can be obtained when therifaximin and one or more amino acids are mixed in aqueous solution. Therifaximin in solution can be raw rifaximin, amorphous rifaximin,rifaximin polymorph or mixtures thereof.

Rifaximin crystals can also be obtained from conglomerates comprisingrifaximin and one or more amino acids. Such crystals can be crystals ofrifaximin β and, such as, for example, crystals of rifaximin β with 3water molecules for each rifaximin molecule (“rifaximin β_(3.0)”); or acrystal of rifaximin β with 4.5 water molecules for each rifaximinmolecule (“rifaximin β_(4.5)”).

Other rifaximin crystals can be obtained by means of drying of solutionscomprising rifaximin and one or more amino acids, or by means of directdrying (e.g. P₂O₅) of crystals of rifaximin β, wherein the size of thesecrystals is large enough to be analyzed by single crystal X-raydiffraction.

On the basis of cell parameters and structural details it is possible tocalculate the theoretical diffractogram which is then compared to theexperimental one obtained from the powders. As described herein,obtained crystals can include crystals of rifaximin α. In someembodiments, the crystals of rifaximin α do not contain any watermolecule (“α₀”). In some embodiments, the crystals of rifaximin α have0.5 water molecules for each rifaximin molecule (“α_(0.5)”). In someembodiments, the crystals of rifaximin α contain 1.5 water molecules foreach rifaximin molecule (“α_(1.5)”).

The crystalline structures of rifaximin β_(4.5), β₃, α_(1.5) and α_(0.5)are hydrates that have the characteristic of containing at least onewater molecule for each dimeric unit, characterized in that the waterinteracts by means of a hydrogen bond with the amide nitrogen atposition 14; this water molecule enters the ansa chain of the rifaximinstructure.

In the microcrystalline powders described in the literature, thediffraction profile is given by the superimposition of the diffractionprofiles of single crystals constituting the powder, each of which ischaracterized by a different content of crystallization water.Therefore, obtaining single rifaximin crystals with a suitable size forstructural analysis (e.g. by means of single crystal X-ray diffraction)can provide useful structural parameters, from which it is then possibleto calculate rifaximin diffractograms corresponding to differentcontents of crystallization water.

Single rifaximin crystals thereby obtained can be used as analyticalstandards in crystallographic analysis for the quantitative andqualitative determination of rifaximin mixtures, even complex ones,wherein crystals characterized by different water contents and by cellparameters within the ranges reported in Tables 1 and 2 are present invariable proportion.

The number and position of water molecules within individual rifaximincrystals influence the parameters of the unit cell as well as theposition of the peaks in the X-ray powder diffractogram. The obtainedcrystals allow determination of the presence of a polymorphic form, evenin complex mixtures.

The parameters characterizing the rifaximin crystals described inExamples 2, 4, 6 and 8 have been obtained in the laboratory by means ofan Oxford Diffraction X'calibur diffractometer with MoKα radiation(λ=0.71073 Å) or by means of an XRD1 line at the Elettra Synchrotron inTrieste.

Rifaximin single crystals are useful in quantitatively and qualitativelydetermining the presence of such polymorphic forms in complex rifaximinmixtures, in production batches and in finished pharmaceuticalcompositions comprising rifaximin, and they can also be useful indetermining the amorphous amount in a powder mixture.

In addition to providing methods of obtaining single rifaximin crystals,the use of amino acids is also advantageous in industrial processes forthe preparation of rifaximin pharmaceutical compositions. For example,the presence of one or more amino acids can increase the solubility ofrifaximin in an aqueous solution, thereby allowing avoidance orreduction of the volume of organic solvents to be used. This hasrelevance for products for human or animal use as the amount of residualorganic solvents in the product can be reduced.

Avoidance or reduction of use of organic solvent can also contribute tothe safety of the industrial process to make pharmaceutical compositionscomprising rifaximin. For example, lower quantities of organic solventscan increase the flash point of an aqueous solution of rifaximin.

Rifaximin compositions in the presence of one or more amino acids,wherein the molar ratio of amino acid to rifaximin is from 1:1 and 10:1,preferably from 1:1 and 5:1, can provide an increase of rifaximinsolubility on an order of magnitude of from 1.5- to 20-fold in aqueoussolutions at room temperature. In some embodiments, the compositionscomprising rifaximin and one or more amino acids can provide an increaseof rifaximin solubility on an order of magnitude of from about 1.1- to10-fold at 37° C., depending upon the selected amino acid(s).

The increase in rifaximin solubility in the presence of one or moreamino acids is observed with raw rifaximin, amorphous rifaximin,rifaximin pure polymorphs or their mixtures in the presence of singleamino acids or their mixtures. It has also been observed that thepresence of amino acids can have a synergic effect on rifaximinsolubility in aqueous solutions in which low percentages of organicsolvents are present. For example, the effect is observed in aqueoussolutions in which organic solvents are present in an amount equal to orless than about 20% (v/v).

As shown in Example 11, the solubility of rifaximin in an ethanol/watersolution 1:4 (v/v) containing amino acids in a molar ratio relative torifaximin from 1:3 and 1:5 is 48 μg/mL. As shown in Examples 2, 4, 6 and8 and 10, rifaximin is solubilized at concentrations which are nearlythousand times higher, thus reaching concentrations higher than 30 mg/mlin solution at elevated temperatures, e.g., 100° C. The possible use ofwater/ethanol solutions with a low ethanol content in the productionprocess of crystalline rifaximin represents an advantage from the pointof view of the process safety. Such solutions have a higher point ofinflammability, also called “flash point”, defined as the minimumtemperature at which, at room pressure, a liquid produces vapors in suchan amount that, together with air, they form a mixture which can flareup or explode. Therefore, the higher the flash point, the safer theprocess.

In the case of 20% ethanol solutions (v/v) in water, as in Example 2,the flash point is 36° C., whereas for 70% ethanol solutions, like thosedescribed in the known art for rifaximin crystallization, the flashpoint decreases to 21° C.

Moreover, another advantage is that in solutions having a low ethanolcontent, rifaximin crystallization can be coupled to crystallization ofamino acids that may be present in the solution, thus allowing theobtainment of both crystal forms as a mixture in the solid state using asingle step.

Different amino acids lead to different available rifaximinconcentrations, thus allowing the modulation of rifaximin solubility.

The Examples describe the preparation of solid compositions comprisingdifferent amino acids in various molar ratios with respect to rifaximin,in form of granules for ready suspension and/or tablets. In someembodiments, preparations of solid compositions are provided, whereinthe compositions comprise rifaximin and tryptophan, serine and histidinein molar ratios from 10 to 1 with respect to the rifaximin.

Embodiments are also directed to compositions in the form of granulesfor ready suspension or for tablet preparations, wherein said granulescan include or are coated with agents for controlling release. Exemplaryagents for controlled release are described supra.

The composition can also be in the form of tablets that are coated witha coating or film to provide controlled release.

Embodiments also relate to methods of increasing the solubility ofrifaximin in a composition, wherein one or more amino acid are comprisedin the composition. In some embodiments, a synergic effect on rifaximinsolubility is observed when the one or more amino acids are provided inthe composition in the presence of low volumes of organic solvents.

The Examples described herein also demonstrate that the effect of aminoacids on increased rifaximin solubility is even higher than the effectgiven under fasting and fed conditions

Example 1 describes the preparation of solid compositions of rifaximinand amino acids wherein different amino acids, such as tryptophan,serine and histidine, are mixed in molar ratios from 1:1 to 5:1 relativeto rifaximin (amorphous form or polymorph α).

Example 2 describes the preparation of rifaximin crystals by means of aprocess comprising the solubilization of composition A of Example 1 in asolution of ethanol/water 1:4 (v/v), wherein rifaximin reaches aconcentration in solution corresponding to 40 mg/ml at elevatedtemperatures, e.g. 100° C. A solid mass, which can be defined as aconglomerate, or an assembly of distinguishable and separable crystalsof rifaximin and amino acids, is obtained by slow evaporation of thesolution.

Example 3 describes the structural characterization, by means of X-raydiffraction from a conventional source or from a synchrotron, ofrifaximin crystals obtained from Example 2. The structural resolutionallows to establish that the analyzed crystals are crystals of rifaximinβ, characterized in that they have 3 and 4.5 water molecules perrifaximin molecule.

Examples 4 and 5 describe the preparation and characterization ofrifaximin crystals by single x-ray diffraction obtained bysolubilization of the composition B of Example 1 in a solution ofethanol/water 1:4 (v/v), wherein rifaximin reaches a concentrationcorresponding to 40 mg/ml at elevated temperatures, e.g., 100° C.Examples 6 and 7 describe the preparation and characterization ofrifaximin crystals obtained from composition C of Example 1.

Examples 8 and 9 describe the preparation and characterization ofrifaximin crystals obtained from composition D of Example 1.

Example 10 describes the preparation of crystals of rifaximin α obtainedby transforming the rifaximin crystal obtained in Example 2 in thepresence of dehydrating agents. The same result can be obtained bydrying the solid mass comprising crystals of rifaximin and amino acidsobtained according to Examples 2, 4, 6, and 8 in the presence ofdehydrating agents or under vacuum and at temperatures between roomtemperature and 40° C.

Comparative Example 11 demonstrates that, in the absence of amino acids,rifaximin in a solution of ethanol/water 1:4 (v/v) reaches a maximumconcentration of 48 μg/ml.

Example 12 shows a comparison of rifaximin solubility in aqueoussolutions with or without amino acids at various temperatures. Rifaximinsolubility in aqueous solutions turns out to be about 3 μg/ml at roomtemperature and about 7 μg/ml at 37° C., whereas in the presence ofamino acids its solubility is higher than 30 μg/mL in both cases. Theseresults disclose preparations of pharmaceutical compositions whereinrifaximin is more available.

Example 13 describes the solubility of the solid mass, defined as aconglomerate of amino acids and crystals of rifaximin, preparedaccording to Example 2, which in buffer solution at pH 6.8 reaches arifaximin concentration corresponding to about 30 μg/ml.

These examples demonstrate that the compositions containing rifaximinand one or more amino acids can lead to higher concentration ofrifaximin in water or aqueous solutions compared to solutions whereinamino acids are not present.

Examples 14 and 15 are comparison studies or rifaximin solubility. Theseexamples provide rifaximin solubility values in water and buffers atroom temperature and at 37° C., in the absence of organic solvents andamino acids.

Example 14 describes rifaximin solubility in water and in buffers at pH4, pH 7 and pH 10. These studies demonstrate that rifaximin issubstantially insoluble in water, in particular at those pH values whichare similar to the physiologic conditions.

Example 15 describes rifaximin solubility obtained by means of thedissolution test of coated tablets and tablets comprising rifaximingastroresistant granules, in solutions simulating intestinal fluidsbefore meals (FaSSIF solutions) and after meals (FeSSIF solutions).

Coated rifaximin tablets (NORMIX®) show rifaximin solubility values inFaSSIF solutions of about 8 μg/ml after 360 minutes, whereas in FeSSIFsolutions rifaximin concentration is about 11 μg/ml after 360 minutes.

The tablets comprising gastroresistant granules in rifaximin show arifaximin solubility in FaSSIF solutions of about 13 μg/ml after 360minutes, whereas in FeSSIF solutions rifaximin concentration is about 20μg/ml after 360 minutes.

The examples of the invention show that the effect exerted by aminoacids on increasing rifaximin solubility exceeds rifaximin solubility ofrifaximin compositions prepared in the absence of amino acids inintestinal fluids. The addition of amino acids to gastroresistant andnon gastroresistant compositions may therefore provide higher levels ofbioavailable rifaximin concentrations, in particular after meals.

Example 1

Preparation of Solid Compositions Comprising Rifaximin and Amino Acids

A rifaximin amount corresponding to 200 mg was mixed in a V-mixertogether with respective amounts of amino acid (AA) as reported in Table3.

TABLE 3 Amino acid Molar ratio Composition (AA) Rifaximin formAA:rifaximin A Tryptophan Rifaximin α 4:1 B Serine Rifaximin α 3:1 CHistidine Rifaximin α 4:1 D Histidine Amorphous 4:1 Rifaximin

The obtained mixture can be stored at room temperature without anyparticular further precaution beside those taken for storing rifaximinor amino acids alone.

Example 2

Preparation of Crystals of Rifaximin 13 Starting from the Composition Aof Example 1

5 ml of a solution formed by ethanol/water in volumetric ratio 1:4 (v/v)were added to composition A of Example 1. The mixture was then heated at100° C. until reaching complete dissolution and left for completesolvent evaporation at room temperature for 4 days. Formation ofrifaximin conglomerates characterized by the contemporary presence ofrifaximin crystals and tryptophan crystals were obtained after solventevaporation.

Example 3

Analysis of Crystals of Rifaximin β3.0 and β4.5 Obtained in Example 2

Rifaximin crystals obtained in Example 2 were separated from amino acidsand measured by X-ray diffraction using:

-   -   a) An X'calibur diffractometer by Oxford Diffraction, provided        with a CCD area detector which uses MoKα radiation (λ=0.71073 Å)        and a graphite monochromator; data were collected at room        temperature. The structures were solved by direct methods using        the SHELX97 program (Sheldrick, 2008) implemented in the WinGX        package (Farrugia, 1999);    -   b) synchrotron ELETTRA (Trieste) at the XRD1 beam line at room        temperature and at 295 K, using the cooling system MARSCH 300.

The structures were solved using the SHELX97 program (Sheldrick, 2008)implemented in the WinGX package (Farrugia, 1999). Table 4 reports thecrystallographic parameters relating to the analyzed rifaximin crystals.

TABLE 4 Crystallographic Crystallographic parameters parameters Chemicalformula C₄₃H₅₇N₃O₁₄ C₄₃H₆₀N₃O_(15.5) H₂O molecules for 3.0 4.5 eachrifaximin molecule MW 839.93 866.95 Temperature/K 295 295 λ(Å) 0.71073 1Crystalline system monoclinic monoclinic Space group P2₁ P2₁ a/Å13.7960(8) 13.753(8) b/Å 19.944(4) 19.749(4) c/Å 16.607(6) 16.378(6)β/deg 92.180(1) 91.972(1) V/Å³ 4566.1 4445.8(6) Z 4 4 D_(c)/Mg m⁻³ 1.2221.295

Based on the known cell parameters and structural details, the crystals(Example 2) were determined to be crystals of rifaximin β, calledrifaximin β_(3.0) and rifaximin β_(4.5).

Example 4

Preparation of Rifaximin Crystals Starting from the Composition B ofExample 1

5 ml of a solution formed by ethanol/water in a volumetric ratio 1:4(v/v) were added to composition B of Example 1. The mixture was thenheated at 100° C. until reaching complete dissolution and left forcomplete solvent evaporation at room temperature for 4 days. Formationof rifaximin crystals and serine crystals was obtained after solventevaporation.

Example 5

Analysis of Rifaximin Crystals Obtained in Example 4

Rifaximin crystals obtained in Example 4 were separated from amino acidscrystals. Cell parameters were determined at room temperature by meansof an X'calibur diffractometer by Oxford Diffraction using the MoKαradiation (λ=0.71073 Å).

Table 5 reports crystallographic parameters relating to the analyzedrifaximin crystal.

TABLE 5 Crystallographic parameters Temperature/K 295 Morphology Orangeprism Crystalline system monoclinic Space group P2₁ a/Å 13.86 (1) b/Å19.90 (1) c/Å 16.69 (1) β/deg 91.85 (1)

Based on the known cell parameters, the analyzed crystal was determinedto be a crystal of rifaximin β.

Example 6

Preparation of Rifaximin Crystals Starting from the Composition C ofExample 1

5 ml of a solution formed by ethanol/water in a volumetric ratio 1:4(v/v) were added to composition C of Example 1. The solution was thenheated at 100° C. until reaching complete dissolution and left at roomtemperature for 4 days for evaporation of the solvent. Formation ofrifaximin crystals and histidine crystals were obtained after solventevaporation.

Example 7

Analysis of Rifaximin Crystals Obtained in Example 6

Rifaximin crystals obtained in Example 6 were separated from amino acidscrystals. Cell parameters were determined at room temperature by meansof an X'calibur diffractometer by Oxford Diffraction using the MoKαradiation (λ=0.71073 Å).

Table 6 reports crystallographic parameters relating to the analyzedrifaximin crystal.

TABLE 6 Crystallographic parameters Temperature/K 295 Morphology Orangeprism Crystalline system monoclinic Space group P2₁ a/Å 13.75 (1) b/Å19.76 (1) c/Å 16.35 (1) β/deg 92.09 (1)

Based on known cell parameters, the analyzed crystal was determined tobe a crystal of rifaximin β.

Example 8

Preparation of Rifaximin Crystals Starting from the Composition D ofExample 1

5 ml of a solution formed by ethanol/water in a volumetric ratio 1:4(v/v) were added to composition D of Example 1. The solution was thenheated at 100° C. until reaching complete dissolution and left at roomtemperature for 4 days for evaporation of the solvent. Formation ofrifaximin crystals and histidine crystals were obtained after solventevaporation.

Example 9

Analysis of Rifaximin Crystals Obtained in Example 8

Rifaximin crystals obtained in Example 8 were separated from amino acidscrystals. Cell parameters were determined at room temperature by meansof an X'calibur diffractometer by Oxford Diffraction using the MoKαradiation (λ=0.71073 Å).

Table 7 reports crystallographic parameters relating to the analyzedrifaximin crystal.

TABLE 7 Crystallographic parameters Temperature/K 295 Morphology Orangeprism Crystalline system monoclinic Space group P2₁ a/Å 13.78 (1) b/Å19.74 (1) c/Å 16.38 (1) β/deg 92.12 (1)

Based on these cell parameters, the analyzed crystal was determined tobe a crystal of rifaximin β.

Example 10

Preparation of Crystals of Rifaximin α₀, α_(0.5) and α_(1.5)

Rifaximin crystals obtained in Example 2 were placed in a dryer at roomtemperature and under ambient pressure in the presence of P₂O₅ for 24hours.

Rifaximin crystals were analyzed by means of X-ray diffraction using:

-   -   a) An X'calibur diffractometer by Oxford Diffraction, provided        with a CCD area detector which uses MoKα radiation (λ=0.71073 Å)        and a graphite monochromator; data were collected at room        temperature. The structures were solved by direct methods using        the SHELX97 program (Sheldrick, 2008) implemented in the WinGX        package (Farrugia, 1999);    -   b) synchrotron ELETTRA (Trieste) at the XRD1 beam line at room        temperature and at 100 K, using the cooling system MARSCH 300.

The structures were solved using the SHELX97 program (Sheldrick, 2008)implemented in the WinGX package (Farrugia, 1999).

Table 8 reports the crystallographic parameters relating to the analyzedrifaximin crystals.

These crystals were characterized in that they are crystals of rifaximinα and in that they have precise water molar ratios.

TABLE 8 Crystal Crystal Crystal Crystal parameters 3 parameters 4parameters 4 parameters 5 Chemical C₄₃H₅₁N₃O₁₁ C₄₃H₅₁N₃O_(11.5)C₄₃H₅₁N₃O_(11.5) C₄₃H₅₄N₃O_(12.5) formula H₂O 0 0.5 0.5 1.5 moleculesfor each rifaximin molecule MW 785.87 794.89 794.89 812.83 temperature/K295 295 100 295 λ(Å) 1 0.71073 1 0.71073 Crystalline monoclinicmonoclinic monoclinic monoclinic system Space group P2₁ P2₁ P2₁ P2₁ a/Å14.232(4) 14.579(4) 14.401(4) 14.492(4) b/Å 19.822(4) 20.232(4)19.662(4) 20.098(4) c/Å 16.164(4) 16.329(4) 16.153(4) 16.215(4) β/deg108.74(3) 111.21(3) 111.04(3) 111.21(3) V/Å³ 4318.2(5) 4402.7(5)4268.6(5) 4402.7(5) Z 4 4 4 4 D_(c)/Mg m⁻³ 1.209 1.237 1.237 1.226

Analogous results were obtained by drying under the same conditionsstarting from rifaximin crystals obtained in Examples 4, 6 and 8.

Example 11

Determination of Rifaximin Solubility in Ethanol-Water Solutions

A rifaximin amount corresponding to 200 mg was dissolved in 10 ml of asolution formed by ethanol/water 1:4 (v/v), left for two days understirring at room temperature.

Rifaximin solubility was determined by wavelength spectrophotometry atwavelength 276 nm with a rifaximin calibration curve. The measuredrifaximin solubility for the sample is 48 μg/ml.

Example 12

Determination of Rifaximin Solubility in Aqueous Solutions in thePresence of Amino Acids

The example describes the determination of rifaximin solubility inaqueous solutions in the presence and in the absence of amino acids.

In particular, experiments are summarized in Table 9.

-   -   Experiment 1 describes the determination of the solubility        obtained by placing 20 mg rifaximin in phosphate buffer at pH        6.8 (P.B.) at room temperature for 2 hours;    -   Experiments 2, 3 and 4 describe rifaximin solubility in water        obtained by placing 200 mg rifaximin and 195 mg tryptophan,        serine and histidine, respectively corresponding to molar ratios        rifaximin:amino acid 1:4, 1:7.5 and 1:5, and the solutions are        kept under stirring at room temperature for 24 hours.    -   Experiments 5-14 describe the solubility obtained by placing 20        g rifaximin in 500 ml buffer at pH 6.8 with various amino acids        in molar ratios reported in Table 9, and the solutions are kept        under stirring for 24 hours at 37° C. in vessel.

Rifaximin solutions are filtered and rifaximin concentrations aredetermined by means of calibration curve spectrophotometry, atwavelength 364 nm.

Table 9 also reports values relating to the increase of rifaximinconcentrations in solutions with different amino acids if compared torespective reference solutions of rifaximin alone in water or phosphatebuffer (Trials 1 and 5).

TABLE 9 Conc. trial V AA/Rifax Conc. N./Conc. Experiment AA (ml) T (°C.) Solution (mol/mol) (μg/ml) trials 1 and 5 1 — 50 r.t. P.B. — 3.5 1 2Tryptophan 5 r.t. H₂O 4:1 35 10 3 Serine 5 r.t. H₂O 7.5:1   5.7 1.6 4Histidine 5 r.t. H₂O 5:1 21.8 6.2 5 — 500 37° C. H₂O — 7 1 6 Tryptophan500 37° C. P.B. 10:1  22 3.1 7 Tryptophan 500 37° C. P.B. 3:1 30 4.2 8Tryptophan 500 37° C. P.B. 1:1 21 3 9 Histidine 500 37° C. P.B. 10:1  162.3 10 Histidine 500 37° C. P.B. 3:1 12 1.7 11 Histidine 500 37° C. P.B.1:1 10 1.4 12 Valine 500 37° C. P.B. 3:1 8 1.1 13 Leucine 500 37° C.P.B. 3:1 8 1.1 14 Isoleucine 500 37° C. P.B. 3:1 10 1.4

Example 13

Determination of Solubility in Water of Conglomerates of Rifaximin andTryptophan Obtained in Example 2

The solubility of conglomerates of rifaximin and tryptophan, obtained inExample 2, was determined by placing 653 mg of this solid in 5 mlphosphate buffer at pH 6.8 at room temperature. The solution was leftfor 24 hours under stirring.

Rifaximin concentration in solution was determined by means ofwavelength spectrophotometry at wavelength 364 nm and is 28 μg/ml.

Example 14

Determination of Rifaximin Solubility in Water and in Buffer Solutionsat Various pH (Comparative Example)

Rifaximin solubility values was determined by placing the rifaximinamounts respectively reported in Table 10 in a volume of 50 ml,respectively of water, phosphate buffer at pH 4, phosphate buffer at pH7 and borate buffer at pH 10.

The suspensions were stored under nitrogen and at 30° C., underconditions of stirring, for 24 hours.

The determination of rifaximin concentration in solution was carried outby means of a chromatographic method under the conditions reported inEuropean Pharmacopoeia Ed. 7.1, 04/2011, paragraph 2362, page 3459 andthe obtained results were reported in Table 10.

TABLE 10 Rifaximin Rifaximin amount concentration Trial Solution (mg)(μg/ml) 1 H₂O 20 3.63 μg/ml 2 Phosphate buffer pH 4 15 4.12 μg/ml 3Phosphate buffer pH 7 20 3.22 μg/ml 4 Borate buffer 65  299 μg/ml pH 10

Example 15

Determination of Rifaximin Dissolution Profile in Coated Tablets andTablets Comprising Gastroresistant Microgranules (Comparative Example)

The dissolution profiles for commercially available coated tabletsNORMIX®, comprising 200 mg rifaximin, and tablets comprising 400 mgrifaximin in gastroresistant microgranules were determined under theconditions reported in European Pharmacopoeia, Ed. 7.1, paragraph 2.9.3,page 256-263. Rifaximin quantitative determination was obtained underthe conditions described in European Pharmacopoeia, Ed. 7.1, 04/2011,paragraph 2362, page 3459.

Dissolution profiles of NORMIX® tablets and of rifaximin tablets ingastroresistant microgranules were determined in solutions at pH 5 andpH 6.5 as well as in FaSSIF solutions and FeSSIF solutions.

The FaSSIF solution contains taurocholate sodium, 3 mM; lecithin, 0.75mM; NaH2PO4, 65 mM, NaCl 85 mM, and purified water up to 1 l, with a pH6.5.

The FeSSIF solution contains taurocholate sodium 15 mM, lecithin 3.75mM, glacial acetic acid 144.05 mM, NaCl 203.18 mM, and purified water upto 1 l, with a pH 5.

The dissolution profiles reported in Table 11 were determined by usingtwo NORMIX® tablets and a rifaximin tablet in gastroresistantmicrogranules, in buffer pH 5, buffer pH 6.5, FaSSIF solution and FeSSIFsolution, for 360 minutes.

The dissolution profiles of the 2 of NORMIX® tablets and of therifaximin tablet in gastroresistant microgranules are reported in Table11.

TABLE 11 Rifaximin tablets in Rifaximin Normix ® gastroresistant tabletsmicrogranules Rifaximin concentration Rifaximin concentration (μg/ml)(μg/ml) Buffer pH 5 6.67 ± 0.29  4.49 ± 0.23 Buffer pH 6.5 6.09 ± 1.01 9.56 ± 0.81 FaSSIF 8.40 ± 0.56 13.78 ± 0.64 FeSSIF 11.73 ± 1.99  20.58± 3.27

Example 16

Preparation of Granules Comprising Rifaximin and Amino Acids

An amount of 200 g of rifaximin, amino acids, hydroxypropylmethylcellulose, fumed silica and talc were mixed in a V mixer for 15min at a speed of 16 rpm. The solid mixtures were loaded in a rollercompactor with an applied pressure of up to 100 bar.

The granule composition is reported in Table 12.

TABLE 12 Granule 1 Granule 2 Granule 3 Component (grams) (grams) (grams)Rifaximin 200 200 200 Tryptophan 195 — — Serine — 75 — Histidine — — 145Hydroxypropyl 84.6 104.6 134.6 methylcellulose Fumed silica 0.4 0.4 0.4Talc 20 20 20

The granules so obtained can be coated with coating film orgastroresistant film and used for suspension or tablet preparations.

Example 17

Preparation of Granules Comprising Rifaximin and Amino Acids

An amount of 200 g rifaximin and amino acids, hydroxypropylmethylcellulose, fumed silica and talc were mixed in a V mixer for 15min at a speed of 16 rpm. The solid mixtures were loaded in a rollercompactor with an applied pressure up to 100 bar.

The granule composition is reported in Table 13.

TABLE 13 Granule 4 Granule 5 Granule 6 Component (grams) (grams) (grams)Rifaximin 200 200 200 Tryptophan 195 195 195 valine 117 — leucine — 131Isoleucine 131 Hydroxypropyl 84.6 104.6 134.6 methylcellulose Fumedsilica 0.4 0.4 0.4 Talc 20 20 20

The granules so obtained can be coated with coating film orgastroresistant film and used for suspension or tablet preparations.

Example 18

Preparation of Sachets Comprising Granules of Amino Acid and Rifaximin

The granules obtained according to Example 16 and 17 were added to theexcipients listed in Table 14, and the resulting solid mixtures weredivided in sachets. The sachet unitary composition is reported in Table14.

TABLE 14 Sachet 1 Sachet 2 Sachet 3 Sachet 4 Sachet 5 Sachet 6 Component(mg) (mg) (mg) (mg) (mg) (mg) Granule 1 Example 16 500 Granule 2 Example16 400 Granule 3 Example 16 500 Granule 4 Example 17 500 Granule 5Example 17 400 Granule 6 Example 17 500 Hydrophobic Colloidal 10 10 1010 10 10 Silica Aspartame 20 20 20 20 20 20 Cherry-flavour 100 100 100100 100 100 Sobitol 3370 3470 3370 3370 3470 3370 TOT 4000 4000 40004000 4000 4000

Example 19

Preparation of Tablets Comprising Rifaximin and Amino Acids.

Rifaximin granule prepared as in Example 16 and 17 were mixed withexcipients, and all of the components were mixed in a V binder. Theresulting mixtures were compressed in a tabletting machine. The tabletswere coated with film coating.

The unitary composition of tablets is reported in Table 15.

TABLE 15 Tablet 1 Tablet 2 Tablet 3 Tablet 4 Tablet 5 Tablet 6Components (mg) (mg) (mg) (mg) (mg) (mg) Granule 1 500 Example 16Granule 2 400 Example 16 Granule 3 500 Example 16 Granule 4 500 Example17 Granule 5 400 Example 17 Granule 6 500 Example 17 Sodium starch 15 1515 glycolate Colloidal silica 1 1 1 Talc 1 1 1 Coating filmHydroxypropyl 5.15 5.15 5.15 5.15 5.15 5.15 methylcellulose Titanium 1.51.5 1.5 1.5 1.5 1.5 dioxide Disodium 0.02 0.02 0.02 0.02 0.02 0.02edetate Propylene 0.5 0.5 0.5 0.5 0.5 0.5 glycol Red iron 0.15 0.15 0.150.15 0.15 0.15 oxide E172

The excipients were sieved and then mixed with rifaximin granules; theresulting mixtures were compressed using a rotary tabletting machineequipped with oblong and tablets were obtained.

The tablets were coated using conventional pan equipment, in order toimprove appearance and achieve taste mask properties.

The tablet can also be coated also with gastroresistant film coating.

Example 20

Preparation of Conglomerates of Rifaximin and Amino Acids

Conglomerate A: Rifaximin—Tryptophan 1:4. A volume of 5 ml ethanol/water1:4 (v/v) solution was added to composition A—Example 1; the mixture wasthen heated at 100′C until complete dissolution and left for completesolvent evaporation at room temperature for 4 days. Rifaximinconglomerates characterized by the contemporary presence of rifaximincrystals and tryptophan crystals were obtained.

Conglomerate B: Rifaximin—Serine 1:3. A volume of 5 ml ethanol/water 1:4(v/v) solution was added to composition B—Example 1. The solution wasthen heated at 100° C. until complete dissolution, and left for completesolvent evaporation at room temperature for 4 days. Rifaximinconglomerate was characterized by the presence of rifaximin crystals andserine crystals.

Conglomerate C: Rifaximin—Histidine 4-1. A volume of 5 ml ethanol/water1:4 (v/v) solution was added to composition C—Example 1. The mixture wasthen heated at 100° C. until reaching complete dissolution, and left forspontaneous evaporation at room temperature for 4 days. Rifaximinconglomerate characterized by the presence of rifaximin crystals andhistidine crystals were obtained.

Conglomerate D: Rifaximin—Histidine 4:1. A volume of 5 ml ethanol/water1:4 (v/v) solution was added to composition D—Example 1. The solutionwas then heated at 100° C. until complete dissolution, and left forspontaneous evaporation at room temperature for 4 days. Rifaximinconglomerate characterized by the presence of rifaximin crystals andhistidine crystals were obtained.

Example 21

Determination of Water Solubility of Rifaximin Conglomerates

The solubility of conglomerates A, obtained according to Example 20, wasdetermined by placing 653 mg of this solid in 5 ml phosphate buffer atpH 6.8 at room temperature. The solution was left for 24 hours understirring.

Rifaximin concentration in solution was determined by spectrophotometricmethod at wavelength 364 nm. The rifaximin concentration was 28 μg/ml.

1. A pharmaceutical composition comprising rifaximin or one of thepharmaceutically acceptable salts thereof and one or more amino acid(s),wherein the molar ratio between the amino acid(s) and rifaximin is from1:1 to 10:1, together with pharmaceutically acceptable excipients. 2.The pharmaceutical composition according to claim 1, wherein the molarratio between the amino acid(s) and rifaximin is from 1:1 to 5:1,
 3. Thepharmaceutical composition according to claim 1, wherein the rifaximinis in the crystalline, polymorphous or amorphous form, in the form of ahydrate or solvate and/or in a mixture thereof.
 4. The pharmaceuticalcomposition according to claim 2, wherein the crystalline form ofrifaximin is selected from i) crystals having monoclinic space group P2₁and cell parameters in the ranges: a: 13.7(1)-13.8(1) Å; b:19.7(1)-19.9(1) Å; c: 16.4(6)-16.6(6) Å; β: 92.1(1)-91.9(1) deg., ii)crystals having the features of i) and having 3 or 4.5 water moleculesfor each rifaximin molecule, iii) crystals having monoclinic space groupP2₁ and cell parameters in the ranges: a: 14.2(1)-14.5(1) Å; b:19.7(1)-20.1(1) Å; c: 16.1(1)-16.2(1) Å; β: 108.7(1)-111.4(1) deg. iv)crystals having the features of iii) and having zero or 0.5 or 1.5 watermolecules for each rifaximin molecule, or v) α, β, γ, or δ.
 5. Thepharmaceutical composition according to claim 1, wherein rifaximin is ina dosage from 20 mg to 1200 mg.
 6. The pharmaceutical compositionaccording to claim 1, wherein the amino acid(s) is/are selected fromaliphatic amino acids, aromatic amino acids, basic amino acids,heterocyclic amino acids, branched amino acids, cyclic amino acids,acidic amino acids, hydroxyl or sulfur containing amino acids, amideamino acids or mixtures thereof.
 7. The pharmaceutical compositionaccording to claim 1, wherein the composition comprises at least onearomatic or heterocyclic amino acid.
 8. The pharmaceutical compositionaccording to claim 1, wherein the amino acid(s) is/are selected fromserine, histidine, tryptophan, valine, leucine or isoleucine.
 9. Thepharmaceutical composition according to claim 1, wherein the aminoacid(s) is/are selected from branched chain amino acids.
 10. Thepharmaceutical composition according to claim 1, comprising rifaximinand valine, leucine or isoleucine in a molar ratio of 10:1 with respectto rifaximin.
 11. The pharmaceutical composition according to claim 1,wherein the pharmaceutically acceptable ingredients include dilutingagents, binding agents, disintegregating agents, lubricating agents,release-controlling polymers or bioadhesive polymers.
 12. Thepharmaceutical composition of claim 11, wherein the diluting agent is atleast one selected from the group of: cellulose, microcrystallinecellulose, calcium phosphate, starch, kaolin, dehydrated calciumsulphate, calcium carbonate, lactose, saccharose, glucose, sorbitol andmannitol.
 13. The pharmaceutical composition of claim 11, wherein thebinding agent is at least one selected from the group of: cellulose,cellulose derivatives, carboxy methyl cellulose, microcrystallinecellulose, hydroxy propyl cellulose, hydroxy ethyl cellulose, hydroxypropyl-methyl cellulose, starches, potato starch, maize starch,partially gelatinized starch, gums, synthetic gum, natural gums,polyvinyl pyrrolidone, polyethylene glycol, gelatin, polyols, propyleneglycol, alginates and sugars.
 14. The pharmaceutical composition ofclaim 11, wherein the disintegrating agent is at least one selected fromthe group of: sodium carboxy methyl cellulose, cross-linked sodiumcarboxy methyl cellulose, polyvinyl pyrrolidone, cross-linked polyvinylpyrrolidone, starch, pre-gelatinized starch and silica.
 15. Thepharmaceutical composition of claim 11, wherein the lubricating agent isat least one selected from the group of: silica, magnesium stearate,calcium stearate, sodium stearyl fumarate, hydrogenated vegetable oils,mineral oils, polyethylene glycols, sodium lauryl sulphate, glycerides,sodium benzoate, glyceryl dibehenate and glycerol stearate.
 16. Thepharmaceutical composition of claim 11, wherein the release-controllingpolymer is at least one selected from the group of: copolymers ofacrylic acid, copolymers of methacrylic acid with an acrylic ormethacrylic ester, polyvinyl acetate phthalate, hydroxy propyl methylcellulose phthalate, cellulose acetate phthalate, Kollicoat®, Eudragit@,Aquateric®, Aqoat®; natural polymers and ethyl cellulose.
 17. Thepharmaceutical composition of claim 11, wherein the bioadhesive polymeris at least one selected from the group of: pectins, zeins, casein,gelatin, albumin, collagen, kitosan, cellulose, dextran, polysaccharidesfrom tamarind seeds, xanthan gum, arabic gum, hyaluronic acid, alginicacid, sodium alginate, polyamides, polycarbonates, polyalkylenes,polyalkylene glycols, polyalkylene oxides, polyalkylene terephthalates,polyvinyl alcohols, polyvinyl ethers, polyvinyl esters, polyvinylpyrrolidone, polysiloxanes, polyurethanes, polystyrenes, polymers ofacrylic acid and methacrylic esters, copolymers of methacrylicacid-ethyl acrylate, polylactides, barbituric polyacids, polyanhydrides,polyorthoesters, methyl cellulose, ethyl cellulose, hydroxy propylcellulose, hydroxy butyl methyl cellulose, cellulose acetate, cellulosepropionate, cellulose acetate butyrate, cellulose acetate phthalate,carboxy methyl cellulose, cellulose triacetate, cellulose sulfate sodiumsalt, polymethyl methacrylate, poly isobutyl acrylate, poly octadecylacrylate, polypropylene, polyethylene glycol, polyethylene oxide,polyethylene terephthalate, polyvinyl acetate, polyvinyl chloride,polystyrene, polyvinyl phenol, carboxylic acids, sulphonic acids,phosphonic acids, and neutral and positively charged amines, amides andimines.
 18. The pharmaceutical composition according to claim 1, whereinthe composition is in the form of a powder, paste, tablet, capsule,ovules cream, foam, suspension, solution, aerosol, granulates, ointmentor suppository suitable for human or animal administration.
 19. Thepharmaceutical composition according to claim 1, wherein the compositionis in a form for oral or vaginal or topical administration.
 20. Thepharmaceutical composition of claim 19, wherein the composition includesrifaximin in an amount from 20 to 1200 mg. 21-51. (canceled)